Christine Kobelt: Summer eczema, a type I allergy in Islandic horses: Kinetics of in vivo-sensitisation of basophilic granulocytes monitored by means of a functional in vitro test (FIT)
In this study Islandic horses raised and kept under different conditions and of different age, with and without clinical symptoms sweet itch (summer eczema) were tested at various seasons for their degree of sensitisation by means of a functional in vitro test (FIT) developed by Susanne Kaul (1998).
Focus was put on following questions to be answered:
1. What about the type I sensitisation in horses, raised and living in an area without Culicoides spp. (Iceland) ?
Twelve adult horses, born and living in Iceland, were studied: They showed measurable amounts of immunoglobulins on the surface of their basophils, capable of inducing a degranulation. All of them were “generally” sensitised with functional antibodies. This is the main precondition for the ability to react with a type I hypersensitivity. Four of the tested horses reacted strongly positive with allergen preparations derived from different insects, excluding Culicoides nubeculosus.
Upon challenge with different doses of Culicoides nubeculosus allergen 2 additional horses out of the twelve showed borderline but clear positive reactions. This might be due to cross reactivities with originary icelandic insects (such as Simulium spp.), for sweet itch inducing Culicoides spp. do not exist in Iceland. Whether these cross reactions might indicate a cross sensitization resulting in enhanced susceptibility to sweet itch once exported to Culicoides areas could be elucidated by exposing them to Culicoides midges.
2. How long will horses with sweet itch stay detectably sensitised, when kept in a Culicoides free region for a longer period of time (Spiekeroog) ?
Even after 15 years on Spiekeroog without any clinical signs 17 affected horses still proved a clearly positive sensitisation of their basophils against Culicoides nubeculosus. They did not loose the principle ability to redevelop sweet itch upon new antigen contact, even after a long time (up to 15 years tested). Furthermore, all of the 26 examined horses showed a “general”
sensitisation, due to the existence of antibodies capable of inducing a degranulation upon
antiserum mediated cross linking without knowing the relevant antigen. The strong seasonal variations of their degree of sensitisation up to a temporary “desensitisation” suggests the idea, that insects with cross-reactive epitopes might exist on that island. Through such a restimulation the production of sensitising immunoglobulins might have maintained. Thus, these horses might not live in complete absence of restimulation of their once developed antibody producing cells, as assumed when starting these investigations. Why, however, none of these animals developed allergic symptoms remains to elucidated.
3. When develop young horses a detectable degree of type I sensitisation if born and raised in an area with Culicoides spp. ?
The 10 foals under study were born in southern Germany. After only a few month of age 2 of them clearly showed a “general” and one even a specific sensitisation of their basophils against Culicoides nubeculosus. Furthermore, 10 yearlings, 12 horses in their second year and another group of 10 three years old horses from the same stud were examined. In contrast to the foals, all other ones were “generally” sensitised, i.e. they bore measurable amounts of functional antibodies on the surface of their basophils. Within all of these three ages groups there were individual animals being specifically sensitised against Culicoides nubeculosus in addition. Compared to their reaction in spring some of them had a decreased FIT-response in autumn. This might be due to a relative decrease of Culicoides specific antibodies on the surface of their basophils, possibly caused by competition with other cell bound immunoglobulins of different specificity. The fact, that some of these horses reacted specifically against Culicoides nubeculosus but did not release any histamine under treatment with antiserum against equine immunoglobulins suggests that the antiserum used did not recognize all isotypes of membrane bound antibodies on basophils. This argues for at least two immunoglobulin isotypes functionally sensitising basophils of the horse
4. What about the type I sensitisation in adult icelandic horses with and without clinical signs of sweet itch during the course of the year ?
Fourteen affected and twelve healthy adult individuals (at least 8 years of age) from one stud in southern Germany were repeatedly tested (every 4 to 6 weeks) throughout one year by means of the FIT. Except a few individuals in September all of these horses were “generally”
sensitised throughout the year. A specific sensitisation against C.n. was seen in all horses with clinical signs of summer eczema at any time of examination. However, also about half of the inconspicous horses reacted positively in the FIT, indicating potent regulatory mechanims preventing these horses from clinical symptoms in spite of a functional degree of sensitisation.
The seasonal kinetics in degree of sensitisation varied amongst the horses without any obvious correlation to seasonal or climatic influences including the time of intense allergen exposition.
In agreement with results mentioned above also these adult horses displayed discordant reactions – mainly in September - regarding negative reactions with the antiserum to bridge membrane bound immunoglobulins on basophils but positive ones with the allergen preparation from Culicoides nubeculosus. Again, this points towards at least two different isotypes on the surface of basophils accounting for type I allergic reactions.
5. How did the functional in vitro test (FIT) for type I allergies prove good under field conditions during the different seasons ?
A striking advantage of the FIT is the parallel detection of the total histamine content in the cells (via physical and antibody mediated release), the “general” sensitisation with functional membrane bound antibodies on basophils irrespective of their antigen specificity plus the quantitative verification of specific sensitizing antibodies reacting with allergen preparations in a dose dependent manner. Only with these synchronized information from one blood sample important new facts as well as indications could be achieved (concerning the results in questions 1 to 4of this summary).
A still remaining deficiency of the FIT is due to the fact, that the antiserum against equine immunoglobulines used up to now, is obviously not detecting all cell-bound functional antibodies responsible for a degranulation of basophils. The use of antibodies, capable of detecting all equine immunoglobulins, including the functional isotypes on basophils, is required.